In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is partially dried on the blanket and then transferred to media. Such a printer avoids the changes in media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In aqueous ink indirect printing, an aqueous ink is jetted onto an intermediate imaging surface, typically called a blanket, and the ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. The intermediate imaging member to which the blanket is mounted is heated to maintain the blanket at temperatures within a range of predetermined temperatures at various positions along the blanket. The temperature of the blanket in the print zone is selected to heat the ink very quickly to begin evaporating some of the water and solvent as soon as the ink impacts the surface of the blanket. Typically, this temperature is at least 40 degrees C. and evaporation commences within milliseconds of the drops hitting the blanket surface. Once the ink drops impact the blanket, the drops also spread. The spreading is conditioned on the blanket temperature, impact velocity, capillary wetting, surface energy, and viscous damping effects of the blanket surface.
When ink is ejected onto a hot blanket, evaporation of the ink causes moisture to enter the air in the print zone between the blanket and the printhead. The amount of moisture introduced into the air is driven by the amount of ink ejected by the printheads in the print zone. The moisture can diffuse across the gap between the printhead and the blanket and condense on the printhead if the temperature of the printhead is sufficiently low. Condensation on a printhead face can interfere with the effective and efficient operation of a printhead.
Heating the printhead to a temperature that discourages condensation also adversely affects the printhead. If an inkjet is not operating at a fairly frequent rate, the ink in a nozzle of an inkjet may dry out and clog the inkjet. Even if the printhead is not heated to avoid condensation, the heat transfer between the hot blanket and the printhead may affect inkjets in the printhead. Specifically, heat transfers from the blanket to the printhead from radiation and convection mechanisms. This heat transfer can cause ink to dry in the nozzles of inkjets that are not operated at a rate that replaces the ink at the nozzle before it dries. Therefore, enabling evaporation of ink on the blanket quickly after impact without negatively affecting the inkjets in the printhead is desirable.